Radiocircuit



Aug. 18, 1931. B, s. MQCUTCHEN ET AL 1,819,298

RADIOCIRCUIT Filed March 18, 1926 2 Sheets-Sheet l @WSN Snom/tots.Brunson $.M9Cu1ohen Carl dSande @am 2 Aug/1s, 1931.

B. s. MccUTcHEN ET A1.-

RADIOCIRGUIT Filed March 18 1926 2 sheets-sneer 2 L. ZI

.u ws nh Wel .md rbh C n, .MS s V. n 01| Sr WC m $3513" www PatentedAug. is, i931I narran stares PATENT ort-ics BRUNSON S. 'LCCUTCHEN, OFNORTH PLAINFIELD, AND CARL V. SANDELL, OF EAST ORANGE, NEW JERSEYRADIOGIRCUIT Application filed March 18, 1926. Serial No. 95,709.

Our invention relates to new and useful improvements in the art of radiofrequency amplification, and it particularly relates to a method ofovercoming variations of elficiency of amplification for differentfrequencies commonly encountered in circuits employed in the receptionand amplification of radio signals.

It is well known that in radio frequency amplifying systems theamplification of -the system is affected by the feedbacks occurringihrough the electro-magnetic coupling from the plate circuit and othersuccessive inductive circuits and through the capacity bel5 tween gridand plate back into the tuned grid circuit. If this feedback energyincreases the total amount of energy suicien-tly to overcome the lossesin the circuit, oscillation will occur. The effect of these feedbackscan be overcome by absorbing an amount of energy which is equal to theincrease in energy supplied by the feedbacks to the tuned circuit. Theeffect of these feedbacks is commonly spoken of as reducing theeffective resistance of the tuned circuit. If

an entra resistance be placed in the tuned circuit such that the FR lossin it is equal to the FR gain from the feedbacks, the effects of thefeedbacks will be cancelled out. This feedback energy is a function offrequency and consequently the energy dissipation of the compensatingresistance should also be a function of frequency.

rEhe common methods of reducing these feedbacks are the use of apotentiometer in connection with the grid circuit or a resistance ofhigh value in some part of the radio frequency circuit. These methodsintroduce a loss which is large enough to control the greatest feedbackat any frequency for which the system is used. Consequently for allother frequencies, a loss is introduced which more than compensates forthe feedback. Another system is the well-known neutrodyne system whichcan neutralize only electro-static feedback. Y

An object of this invention is to provide an automatic method forabsorbing the feedback energy, both electro-magnetic and electrostatic,which will absorb just enough energy at each frequency to cancel outcompletely, the effect of the feedback or modify the effect in anydesired manner.

` One of the objects of our invention is to accomplish the above desiredresult without appreciably affecting the normal tuning characteristicsof the system. It is also important that the proposed compensatingnetwork need not be interposed directly in the resonant circuit.

An additional object of our invention is to provide a network that maybe used in a great number of circuit relations to effectively oppose thevariations with frequency, 1n the efficiency of amplification withoutreducing the sensitivity of the system.

Another object of our invention is to provide a network that will havean inherent frequency loss characteristic in its association with thecircuit, and that will require only initial adjustment for any circuitor tube with which it is used.

An additional object of the invention is to provide a compensatingnetwork of impcdance devices that will not require careful design oradjustment and that are admirably suited to commercial manufacture.

An additional object of the invention is to so modify thecharacteristics of an amplifying system as to obtain a maximum amplifihcation at all frequencies without permitting the system to oscillate atany frequency, by absorbing just enough energy at each frequency toprevent oscillation.

Another object of this invention is to so modify the amplification ateach frequency as to produce any desired frequency gain characteristicin the system, i. e. the relationship between the amplification of thesystem and the frequency for which the system is tuned.

Figure l of the accompanying drawings is a diagrammatic view of a radiofrequency amplification system showing a characteristic application ofour invention. Y

Fig. 2 shows characteristic curves that diagrammatically illustrate therelation of the amplification of a radio frequency stage to the turnedfrequency of the system.

Fig. 3 is a diagrammatic view of a radio frequency amplification systemillustrating the use of an energy absorbing network of the characterherein described in coupled relation with the tuned circuit.

Fig. i is a diagrammatic illustration of a radio frequency systemembodying the invention in which a series-parallel network is insertedin the tuned circuit in accordance with the invention.

Fig. 5 is a diagrammatic view of a circuit showing one application of anetwork of the proposed character to the grid circuit of the vacuumtube.

Figs. 6, 7, 8 and 9 diagrammatically illustrate other applications ofthe invention to the plate circuits of radio amplification systems.

The operation of the invention can be illustrated by referring to thespecific circuit of Fig. l. This circuit, to which our invention isapplied, is diagrammatically shown. The conventional parts of thecircuit are indicated by conventional characters. A. signal receivinginductance L2 is connected in a normal manner to the grid and filamentof a vacuum tube. An adjustable tuning condenser C2, for tuning thereceiving circuit to different signal frequencies, is connected in shuntwith the inductance L1. A network Z1 and Z2 of the character hereinproposed is connected across the grid and filament of the vacuum tube asshown. The plate 3 is connected to the remain( er of the system which isrepresented by the conventional symbol L2 as will be understood oy thoseskilled in the art.

In this circuit, the term L2 will be used to represent the combinedself-inductance of the plate circuit and the mutual inductance of theplate circuit causedby the circuit with which the plate circuit iselectrically and magnetically coupled. ln accordance with well-knownvacuum tube theory, this inductance L2 causes an in-phase voltage to befed back through the plate grid capacity, which we will hereinconventionally refer to as C2, and applied to the grid of the tube. Themagnitude of this voltage will depend on the values of L2 and C2, thefrequency, and the impedance between the grid and filament of the tube.Further, in accordance with weliknown vacuum tube theory, the tube willgo into oscillation when the voltage thus fed back through the gridreaches a certain magnitude. in addition to the above mentionedelectro-static voltage feedback, an in-phase electro-magnetic feedbackmay take place from any of the successive coils embodied in L2 to thetiming circuit, depending upon the geometrical relation of the coils andwiring and the frequency. rlhis form of feedback may also be a cause ofor give assistance in setting up objectionable oscillation in the tubecircuit.

In accordance with our invention, as applied to the circuit shown inFig. l, we propose to connect a network consisting of the resistance Z1and the capacity Z2 between the grid and the filament. The function ofthe impedances Zl and Z2 is to equalize the amplification of the entirecircuit over the desired frequency range. Zl being a resistance doesnotvary with frequency. Z2 being a condenser, its impedance varies withfrequency. Consequently the amount of current through the network Z1Z2varies as the frequency is varied, and therefore the energy dissipatedin the resistance is a function of frequency. In general, the amount offeedback energy increases as the frequency to which the circuit is tunedincreases. In this circuit, as the frequency increases, the reactanceoffered by Z2 becomes smaller and consequently more current flowsthrough Z1Z2 and the PB. loss in the resistance of the network becomesgreater. It is therefore obvious that sufficient energy may bedissipated in the network Z1Z2 to substantially overcome 'he effect ofthe feedback energy.

The operation of the tube under these conditions may best be explainedby referring to the accompanying conventional gain characteristic curvesshown in Fig. 2. Curve A is a characteristic curve illustrative of therelation of the amplification of a radio frcquency stage to the tunedfrequency of the. receiving circuit. In this case it is assumed that theinductance of the plate circuit is so small that no oscillation willtake place therefrom the desired frequency rang-e. Curve B is the samething with a higher plate circuit inductance. The intersection of curveB with the curve C shows the frequency at which the particular amplifierwill go into oscillation. Curve D represents the energy loss put intothe system by the introduction of impedances Zl and Z2 into the circuit.Curve E is a summation curve of B and D showing` the net amplificationthat can be obtained when our invention is applied to the circuit havingthe same plate circuit inductance as used in obtaining curve B.

It will be seen from an inspection of curve E that the result of theintroduction of the impedances Zl and Z2 in Fig, l has been tosubstantially equalize the amplification over a wide frequency range.

By the proper choosing of these impedances, curve E can be made to takeany position below the oscillation point. ln other words, it can be madeto vcancel out all the feedback, only a part of it, or to cause anactual loss at all frequencies.

lVhen only a part of the feedback energy is dissipated, the system is ina sensitive regenerative condition at all frequencies.

vThe value of the impedances of such networks can be calculated inaccordance with the following mathematical deductions:

Referring again. to the system shown inf 'mi tion will take place.

relance voltage applied to the grid of the tube ma"I then be representedby the equation v 7 e *Il (01W) where W=21rX frequency and I1 representsthe current of the tuned circuit. The relation between the inducedvoltage of the tuned circuit and t-he resistance R of the circuit may beexpressed by the equation E=RI1- Now if it is assumed that the platecircuit of the tube is so arranged that in-phase feedbacks occur fromplate to the grid of the tube and through mutual inductance between L2and L1, then these feedbacks will produce, in effect, voltages in thetimed circuit L1Cl which Will add to the impressed voltage andconsequently result in an increase (l2) in the current in the tun-edcircuit where l2 is the additional component of current that is causedto flow through this circuit because of the voltage induced into the'circuit from the feedbacks.

If, therefore, the resultant voltage induced into the circuit throughmagnetic feedback be represented by En, and the resulting voltageinduced into the circuit by the electrostatic feedback be represented byFic, then the total induced voltage of the circuit may be represented bythe conventional equation The effect of the component l2 is to produceay larger grid voltage and, hence, if the feedback voltages aresufficiently strong, oscilla- If we then introduce a resista-nce r intothe tuned circuit, as has previously been done in the prior art, we canfor any given frequency and for any given conditions of electro-staticand electro-magnetic feedback, adjust this resistance to such a valuefor given circuit conditions that the total resultant current (I3)flowing through the circuit L1C1 will be reduced until [3:]1. Hence thegrid voltage in this case will be the saine as the grid Vparticulargiven condition, but not for other conditions. For this given relation,the value of the resistance 11 necessary to effect this compensation maythen be expressed by the equation,

when R is the resistance of the tuned circuit for the selectedconditions.

It can be shown that both the electro-static and the electroemagneticfeedback voltages are functions of the frequency for which the circuitL1 C1 is tuned. kThe value of r which will just cancel the feedbackenergy must therefore be changed as the resonant' frequency is changed.Then r for each frequency absorbs an amount of energy which is ust equalto the increase in energy supplied to the tuned circuit by theelectrostatic and electro-magnetic feedback voltages. This does however,require manual adjustment of the resistance r for the cancellation ofthe energy feedback for different frequencies.

Obviously r can therefore be replaced by any energy absorbing systemthat will absorb an amount of energyT equal Ato that absorbed by r asexpressed in Equation (l).

This energyV absorbing system need not be inside the tuned circuit. ltmight be a shunt circuit as illust-rated in Fig. l or it might be acoupled circuit as shown in Fig. 3 and it may also be a series parallelnetwork inserted in the tuned Vcircuit as shown in F i0. a, or theseelements and functions might be incorporated within the tube itself.

In the circuit shown in Fig. l the current of the tuned circuit l1 willdivide between the two branches of the network in a proportion dependingupon the relative 1nagnitudes of the impedances Z1 and Z2. If Z1includes a resistance and Z2 is a complex impedance, Z2 will vary withfrequency. Therefore, the relative portions of the current passingthrough Z1 and Z2 will vary as the frequency is changed, thereby causingthe current through the resistance to change. As the current through theresistance changes, the energy used up in th-e resistance changes as afunction of the frequency. Consequently, the network Z1, Z2 is an energyabsorbing system which varies automatically with frequency and can byproper choosing of its elements be made equivalent in energy absorptionto the value of 1 derived in Equation (l).

In general this absorbing circuit/may be associated with any portion ofthe system through which energy flows. lt is thus not limited to thegrid circuit and may be used in the plate circuit, with an auxiliarycircuit coupled to the plate or the grid circuit, or in auto transformerconnections associated with either plate or grid circuit or may beincorporated inside the tube.

An interesting development of the circuit of Fig. l occurs when the gridand filament capacity is lumped with the impedance Z2. The circuit thenbecomes that of Fig. 5. Using this circuit, a particular broadcastamplifier was found to normally oscillate up to 450 meters beforeintroducing the network Z1 Z2 into the circuit. When the network shownin Fig. 5 was introduced, however, Z1 being a resistance of about 100ohms, and Z2 being a condenser of about 50 micromicrofarads theamplifier was stable over the whole broadcast range without appreciableloss in efficiency above 450 meters.

The various figures of the accompanying drawings show differentapplications of the involved principles to tuned signal receivingcircuits which applications are to be considered as illustrative only ofour invention. Like numerals and letters in each ease represent the samequalities of the apparatus, the particular values of the elements of thenetwork being dependent upon the relations of the remainder of thecircuit.

As previously explained, the energy absorbing network may also beinserted in the plate circuit.

Figs. 6, 7'. 8 and 9 illustrate the respective plate circuits that areanalogous to the respective grid circuit applications of the inventionalready described, namely Figs. l, 3, 4 and 5.

It should be understood that the particular systems herein described andshown are merely for the purpose of illustrating and eX- plaining ourinvention and are not intended as a complete exposition of all of theapplications of the invention.

lVhat we claim is:

l. The combination with a radio frequency amplifying` system, of acircuit including an energy absorption network, energy absorbing .meansincluded in said network and means inversely responsive to frequency forbypassing portions of the reproducing current around said energyabsorbing means and for causing other portions of said reproducing`current, the latter being substantially proportional to the frequency ofthe system, to traverse said energy absorbing means whereby theabsorption of energy by said network substantially equals theregenerative electromagnetic and electro-static feedbacks in the systemat substantially all frequencies within the normal operating range ofthe system.

2. The combination with a radio frequency amplifying system, of acircuit including an energy absorption network comprising a plurality ofbranches, energy absorbing means in one of said branches and meansincluded in the network for by-passing predetermined portions of thereproducing current around said energy absorbing means and for causingother portions of said reproducing current to traverse said energyabsorbing means, the portions ofsaid current traversing said energyabsorbing means being a predetermined function of the total regenerativeelectro-magnetic and electro-static feedbacks in the system atsubstantially all normal operating frequencies of the system whereby thefrequency gain characteristic of the system is controlled for all normaloperating frequencies thereof.

3. In a radio frequency amplifying system, the combination with aconductor carrying the amplifying current, of an intel'- posed energydissipated network embodying a plurality of branches, energy absorptionmeans included in one of said branches of said network and meansincluded in other portions of said network for by-passing portions ofthe reproducing current around said energy absorption means, theby-passed current portions being inverse functions of thecumulative.electro-magnetic and the electrostatic feedbacks of thesystem whereby the current traversing the energy absorption branch ofthe network is modified in predetermined proportion to the main currentand in accordance with the frequency of the system, thereby producingpredeterminedv desired modifications of the frequency gaincharacteristic of the system.

Ll. The combination with a radio frequency amplifying system of anon-resonant network comprising a circuit containing a resistanceelement capable of dissipating electrical energy in the form of heat anda second circuit that controls the flow of current through theresistance element of the first named circuit, and that contains anelement whose impedance is reactive at all frequencies within the rangeof the amplifying system, the resistance element of the first circuitbeing rclated .to the reactance element of the second circuit so thatthe electrical energy dissipated in said resistance element in the formof heat is a function of the signal frequency which is being amplified.

5. The combination with a radio frequency amplifying system of anon-resonant network for preventing oscillation in a radio frequencyamplifier, comprising a circuit containing a resistance element, and asecond circuit containing an impedance of such a nature as to bereactive at all frequencies within the range of the amplifying system,whereby the flow of current through the resistance element of the firstcircuit is controlled, the amount of energy dissipated in the resistanceelement of the first circuit being a function of the frequency.

ln testimony whereof we affix our signatures.

BRUNSON S. MCCUTCHEN.

CARL V. SANDELL.

